The roof of an underground mine is secured using rock bolts to confine the rock. The rock bolts are inserted into a hole, predrilled into the rock.
Usually, the rock bolt is secured at the top of the hole by a mechanical or chemical anchor creating a point anchor. Alternatively, or additionally, the rock bolt is secured in the hole by grouting along its length with a cement or polyester resin.
Grouting of the bolt along its entire length is the much preferred method to improve confinement of the rock through load transfer between the rock and bolt along its entire length rather than at a local area only. Grouting of the bolt also protects it from corrosion.
In rock bolt installations where it is feasible to drill a relatively small diameter hole (up to 32 mm diameter) in the rock face, it is preferred to use chemical anchors at the top of the hole which extrude the resin grout along the length of the bolt so as to effectively fully grout the bolt.
In installations where larger diameter holes (typically 45 mm diameter) need to be drilled, resin cartridge chemical anchors are relatively ineffective as the large annulus between the conventional solid bolt (typically 22 mm diameter) and the hole wall is too large to enable effective mixing of the resin. In such cases, the bolts are typically post-grouted with a cement grout to fully encapsulate the rock bolts.
Australian Patent Application No. 49856/93 discloses a method of post-grouting a rock bolt which utilises a plastic sleeve mounted on the bolt and which extends along substantially the entire length of the bolt (approximately 2 m long) toward the top of the hole. The sleeve communicates with a grouting chamber located at the base of the bolt below the roof plate. Grout is pumped into the grouting chamber, using a grout nozzle pushed into a hole provided in the grouting chamber. The grout flows upwardly through the inner annular passage defined between the sleeve and rock bolt to the end of the sleeve at the top of the hole and then spreads down the outer annular passage between the sleeve and the wall of the rock bolt hole. The plastic sleeve accordingly divides the grout into two annular columns, which provides additional corrosion protection to the rock bolt. A tensioning nut is threaded onto the exposed end of the bolt and engages the body of the grouting chamber which in turn engages a roof plate bearing against the rock face surrounding the rock bolt hole.
The above method, however, suffers from several disadvantages. Firstly, when the rock bolt is installed into a roof covered with a wire mesh, the leading end of the plastic sleeve often catches on the wire, obstructing the installation. Secondly, once the grouting nozzle is detached from the grouting chamber, grout drains from the grouting chamber under gravity through the hole into which the grouting nozzle had been inserted, thereby reducing the level of grout encapsulation of the bolt. Thirdly, whilst the grout encapsulates the rock bolt within the rock bolt hole, the tail end of the rock bolt and the nut used to pre-tension the rock bolt remain exposed and subject to corrosion. Further, if the rock face is uneven, the roof plate will often not be perpendicular to the hole and the rock bolt when pressed against the rock face by tensioning of the nut. This causes the grouting chamber body, moving with the roof plate as a result of friction, to be misaligned with the nut. Accordingly, the nut and grouting chamber body do not engage with parallel faces and the ability to tension the rock bolt with the nut is thereby reduced.